You have the Compute Module versions of the pi with embedded storage.
The reasoning behind the SD cards on the consumer pis is that they are quick to swap out, and don't have the problems that "bricking" the OS could get you into with booting from onboard storage. There is the possibility to use USB booting in the latest boards (certainly with the 3+ not sure about the status of the 4) which has a flag set in the onboard eeprom so you can use external SSDs/USB drives.
The reasoning behind the SD cards on the consumer pis is that they are quick to swap out, and don't have the problems that "bricking" the OS could get you into with booting from onboard storage. There is the possibility to use USB booting in the latest boards (certainly with the 3+ not sure about the status of the 4) which has a flag set in the onboard eeprom so you can use external SSDs/USB drives.